1. Field of the Invention
The present invention relates to a redundant changeover apparatus, and in particular to a redundant changeover apparatus which makes a changeover from a working system to a protection (standby) system when a line failure or a device failure arises, or when a manual changeover is performed in node devices forming a network where transmission lines (optical fibers) are connected in a ring shape.
2. Description of the Related Art
Heretofore, with progresses of optical transmission technology, a wavelength division multiplexing (WDM) technology has been widely used, enabling a plurality of signals by using lights whose wavelength are different from each other to be multiplexed to realize a network capable of a higher transmission and extensively utilized for a transmission line of ITU Recommendation G.707 SONET/SDH device.
An optical ring network using such a WDM technology is shown in FIG. 15. This optical ring network RNW is composed of, in this example, node devices 10-1 to 10-8, among which only the node device 10-6 forms a relaying or repeating node device and the other node devices 10-1 to 10-5, 10-7, and 10-8 respectively form a multiplexing node device.
The multiplexing node devices 10-1 to 10-3 form a network “A” together with transmission devices 11-1 to 11-3 which are also node devices connected thereto respectively. The multiplexing node devices 10-4 and 10-5 form a network “B” together with transmission devices (node devices) 11-4 and 11-5 connected thereto respectively.
Furthermore, the multiplexing node devices 10-7 and 10-8 form a SONET/SDH ring network “C” together with SONET/SDH transmission devices (node devices) 11-6 and 11-7 connected thereto respectively.
It is to be noted that the multiplexing node devices and the transmission devices are occasionally represented by general reference numerals “10” and “11”, respectively.
The node devices 10-1 to 10-8 are mutually connected through transmission lines “W” of a working system and transmission lines “P” of a protection system which are respectively duplicated, in which each of the multiplexing node devices 10-1 to 10-5, 10-7, and 10-8 is composed of a transmission line monitor 20, a WDM device 30 performing optical coupling/branching per wavelength, and a redundant changeover apparatus 40 performing a redundant changeover at a network single wavelength channel line, as exemplified shown only in the multiplexing node device 10-5, to perform a wavelength division multiplexing of a subordinate network transmission signal at the WDM device 30 and to perform a failure monitoring of the transmission lines W and P at the transmission line monitor 20. The WDM device 30 is connected to the transmission device 11-5 through the redundant changeover apparatus 40 whereby upon a failure (fault) of the working transmission line W, the redundant changeover apparatus 40 performs the redundant changeover to the protection transmission line P to secure a connection with the transmission device 11-5 for achieving the protection of the transmission lines failed.
Also in the relaying node device 10-6, although not shown, the redundant changeover apparatus 40 similarly performs a changeover from the working transmission line W to the protection transmission line P for the relaying operation.
FIG. 16 shows a prior art example of the redundant changeover apparatus 40 in such a node device.
The redundant changeover apparatus 40 is composed of a frame terminal portion 1, a frequency generator 2, a changeover portion 3, and a frame generator 6.
The frame terminal portion 1 comprises a frame terminal unit 1-1 and a frame synchronizer 1-2 for terminating an overhead and an error correction code of a frame from the signal of the working transmission line W to extract transmission signals from the payload, so that from the frame synchronizer 1-2 a detection result 1-3 as to the synchronization is outputted.
This frame terminal portion 1 is similarly composed of a frame terminal unit 1-4 and a frame synchronizer 1-5 for the protection transmission line P.
The frequency generator 2 comprises a transmission frequency clock extractor 2-1 and a PLL (Phase Locked Loop) circuit 2-2 for the signals of the working transmission line W, and comprises a transmission frequency clock extractor 2-3 and a PLL circuit 2-4 for the signals of the protection transmission line P, so that based on the output signals of the PLL circuits 2-2 and 2-4, the frame synchronizers 1-2 and 1-5 respectively detect the presence/absence of the frame synchronization.
The changeover portion 3 comprises a changeover switches 3-11 and 3-12, in which a working system contact Sw of the changeover switch 3-11 is connected to the frame terminal unit 1-1 and the transmission frequency clock extractor 2-1, and a protection system contact Sp of same is connected to the frame terminal unit 1-4 and the transmission frequency clock extractor 2-3.
Also, a working system contact Sw of the changeover switch 3-12 is connected to the transmission frequency clock extractor 2-1 and a protection system contact Sp of same is connected to the transmission frequency clock extractor 2-3.
The frame generator 6 comprises a frame adder 6-1 which inputs the output signal of the changeover switch 3-11 to be inserted with additional bits such as a frame synchronizing signal, and a frame generating unit 6-2 which inputs the output signal of the changeover switch 3-12 for the generation of a frame to be provided to the frame adder 6-1.
A WDM transmission signal outputted from the redundant changeover apparatus 40 is subject to a frame synchronization by the frame terminal portion 51 comprising a frame terminal unit 52 and a frame synchronizer 53 in a receiver 50.
In such a conventional ring network, a synchronization network is mainstream having a master clock source, from which clocks are provided to the node devices, thereby avoiding synchronization loss upon the redundant changeover.
On the other hand, recently, an asynchronous network is going mainstream so that master clocks may not spread over the node devices, whereas since a transparent transmission for each working system/protection system between node devices would be conducted in an asynchronous mode, changeover operations between the working system and the protection system in a failure such as a transmission failure or a device failure will be followed by instantaneous clock changeovers.
Therefore, it was disadvantageous that the receiver (see FIG. 16) causes therein a frame synchronization loss in frequency and phase. This will be described hereinbelow referring to FIG. 17.
It is now supposed that the phases of a working system clock shown in FIG. 17A and a working system frame shown in FIG. 17B are outside phases of a protection system clock shown in FIG. 17D and a protection system frame shown in FIG. 17C.
Namely, as shown in FIG. 15, the node devices 10-1 to 10-3 and the transmission devices 11-1 to 11-3 forming the network “A” are synchronized with each other, and the node devices 10-4 as well as 10-5 and the transmission devices 11-4 as well as 11-5 forming the network “B” are also synchronized with each other.
Also, the multiplexing node devices 10-7 and 10-8 as well as the transmission devices 11-6 and 11-7 forming the SONET/SDH ring network “C” are mutually in the synchronized state with each other.
However, in many cases, the networks “A”, “B”, and the SONET/SDH ring network “C” are asynchronous with each other, so that a phase shift or deviation as shown in FIGS. 17A-17D will arise.
Accordingly, when the phase of the working system frame shown in FIG. 17B is instantaneously changed over to that of the protection system frame shown in FIG. 17C by the changeover switches 3-11 and 3-12, the phase of the overhead OH in the WDM transmission signal shown in FIG. 17E outputted from the frame generator 6 is deviated and provided to the receiver 50, in which since there exists no overhead OH at the phase of a receiver clock RCK expected by the frame synchronizer 53, a frame synchronization loss will arise.
Resultantly, this synchronization loss will extend to synchronizations losses {circle around (1)}-{circle around (9)} counterclockwise from the node device 10-5 where a changeover has arisen, as shown in FIG. 18, and eventually to the network RNW in its entirety.
Additionally, the re-synchronization of frame requires time for frame synchronization-protecting stages, and the sequence starting from the node device 10-5.
The time for establishing the re-synchronization of all network nodes within the ring is an accumulation of the protection time, requiring a changeover time corresponding to the number of node devices×the protection time, resulting in a practically difficult operation.